Change in the thickness of a paint layer with time relates to the amount of volatilization of solvents contained therein and the change in structure of each material contained. Therefore, the measurement of the change in thickness of the paint layer is useful for evaluating the properties of the paint and also for determining the best conditions for painting such as the thickness of the paint layer to be applied.
Although there have been proposed many devices for measuring the change in thickness of a paint layer, it is difficult to say that they have enough functions to get the user's approval.
For example, there has been proposed a measuring device capable of measuring the thickness of the paint layer by direct contact therewith. However, the device does not have wide use, because it is useful only when the surface of the work to be painted is flat and only after the paint layer has been hardened sufficiently. Moreover, the device has the disadvantages that it is difficult to use because the contact pressure upon the paint layer must be kept at a precise constant value and that the paint layer is destroyed at the measuring points by said direct contact therewith. In other words, a device of this type is impossible to use for the measurement of the change in thickness of a paint layer which has not yet hardened sufficiently.
Also, all measuring devices previously proposed have the common disadvantage that they cannot measure the change in thickness of a paint layer time sequentially or the behavior thereof under the same conditions of the actual air flow to which the paint layer may be exposed, because they have no means for controlling such conditions as the velocity, the temperature and the humidity of the air flow which might affect the change of the paint layer.